How do I get my sister to break up with her boyfriend?

My sister is dating this complete loser. She is 17 and her boyfriend is 21. He just got his GED last month, he doesn't have a licenc\se and doesn't have a job. He drinks and does drugs. And it still drive me nuts he never gave her a christmas present. Not even like a letter, he gave her nothing. Actually the only thing he has given her is a STD. They have been dating since September and I've been trying to find what is good in him. He must be good in bed or something. But I can't stand seeing them fight all the time. Is there anything I can do the help them split apart?
How do I get my sister to break up with her boyfriend?
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